This invention relates to scintillation detection system, and is more particularly directed to apparatus for enabling conventional beta-counting liquid scintillation systems to count high-energy radiation, such as gamma rays, emitted by samples carried in standard laboratory test tubes.
Beta-counting systems are employed by many laboratories in clinical testing procedures. In certain procedures, such as radioimmunoassay (RIA), it is desired to measure gamma rays or X-rays, such as the emissions from radioiodinated substances in biological samples. Because of the high cost of commercially available gamma-counting systems it has previously been proposed to measure gamma rays and other high-energy emissions with existing beta-counting systems. See, for example, Clinical Briefs, Nos. 1-4 and Nos. 6 and 7, published by Beckman Instruments, Inc., Fullerton, California. In a first method described in Clinical Briefs the gamma-emitting sample is mixed with a liquid scintillator contained in a small sample vial which is carried in a larger vial holder. In a second method, the gamma-emitting sample is placed in a small plastic tube depending from the cap of a standard vial into liquid scintillator contained in the vial. These methods have certain disadvantages. For example, both methods require that the samples be transferred from the standard laboratory test tubes in which they have been developed to separate sample containers, a procedure which runs the risk of spilling the samples. In the first method, since the sample is directly mixed with the liquid scintillator, the sample is sacrificed and the liquid scintillator is rendered unfit for further use. In the second method, the sample-holding tube is contaminated by the sample, and it has been found that radioactive substances present in certain samples can diffuse through the wall of the plastic tube to contaminate the liquid scintillator.